1. Field of the Invention
This invention relates to a ladder type piezo-electric filter typically used as a filter circuit in a radio communication unit such as, for example, a portable radio communication unit or a mobile automobile telephone, and, more particularly, relates to a ladder type piezo-electric filter used in a high frequency band filter operating at frequencies higher than 1 MHz.
2. Discussion of the Related Art
Nearly all conventional ladder type piezo-electric filters used in the aforementioned radio communication applications operate at a response frequency of 455 KHz. In the conventional ladder type piezo-electric filter, as shown in FIG. 6 of the appended drawings, series piezo-electric resonators (S.sub.1, S.sub.2 and S.sub.3) having a large thickness, a small capacitance, and a resonance frequency of 455 KHz are connected in a series/parallel ladder configuration with parallel piezo-electric resonators (P.sub.1, P.sub.2, and P.sub.3) having a small thickness, a large capacitance and an anti-resonance frequency of 455 KHz.
A demand presently exists in radio communication units and mobile automobile telephone applications in particular for ladder type piezo-electric filters operating at a high frequency such as, for example, 1.7 MHz or 2 MHz. However, heretofore, it has not been practical to use conventional ladder type piezo-electric filters in these applications because the piezo-electric resonators making up the filter have dimensions in the order of 5 mm. 3 and a resonate frequency of 455 KHz. In contrast, the aforementioned applications require higher resonant frequencies and dimensions of 2 mm or less.
A conventional disk-shaped or rectangular-plate-shaped piezo-electric resonator, 5 mm in dimension and oscillating in a circumferential direction, typically provides a fundamental mode (FM) having a high resonance impedance near 500 KHz, and a second harmonic mode (SM) and a third harmonic mode (TM) as shown in FIGS. 7 and 8. FIG. 7 plots electrical impedance (.OMEGA.) as a function of resonant frequency (kHz). FIG. 8 plots electrical charge (+or -) as a function of position measure in the circumferential direction on the surface of the conventional piezo-electric resonator for (FM), (SM) and (TM). The conventional 455 KHz resonator utilizes the fundamental mode (FM). As shown in FIG. 7, the second (SM) and third harmonic modes (TM) of the conventional resonator appear at frequencies higher than 1 MHz. Use of these second and third harmonic modes has been considered, but has not been practical because the respective resonance impedances of the harmonic modes are less than that of the fundamental mode.